A fastening system for a plate-shaped structural element on a pitched roof is provided. The plate-shaped structural element is bordered in a water-tight manner in a frame and fully replaces the roof covering, and the eaves-side and ridge-side cross beams of the frame which run transversely to the ridge-eaves direction have profiles which are matched to each other and engage in each other. The top side of the eaves-side cross beam has a mounting that surrounds the edge of the plate-shaped structural element, and the bottom side thereof has a support for fastening to the roof structure. The support includes a web that extends from the mounting to the support, a protrusion extending beneath the mounting and receives an extension that extends from the ridge-side cross beam thereof.